Remove now-unused config_file::is_empty()

Its only call site was the is_empty() gate removed from ux-window.cpp's
grid_overlay backfill in the previous commit. Flagged by the rs-agentic
bot's automated review of that push; confirmed zero remaining callers.

Author: Evgeni Raikhel

common/rs-config.h

@@ -102,8 +102,6 @@ namespace rs2
 
         void remove(const char* key);
 
-        bool is_empty() const { return _j.empty(); }
-
         static config_file& instance();
 
         // Retrieves a value from a nested JSON structure using dot notation